The present invention relates to chromatography systems used for chemical analysis and, more particularly, to an improved septum for an injection port of a gas chromatography system.
In gas chromatography, samples are separated into their components by passing the sample through a separating column. The sample is introduced into a flowing carrier gas in an injection port. The carrier gas sweeps the sample from the injection port into the separating column. The separated components emerging from the column are eluted through a detector that monitors the elution over time. A chromatogram, representing eluting quantity as a function of time, is generated by plotting the detector output signal.
Typically, the carrier gas is sealed from the outside world by a rubber septum. The sample is introduced into the injection port using a syringe that pierces the septum. The sample is injected into the inlet system and the syringe needle is withdrawn.
The septum must serve two functions. First, it must form a seal around the needle to prevent leaks while the injection is effected. Second, it must reseal the injection port after the syringe needle has been withdrawn and maintain this seal while the chromatographic separation is being executed. A leak occurring while the needle is in the septum can cause part of the injected sample to escape the injection system. This "injection" leak is difficult to detect because it occurs during the dynamic process of injection and is only apparent by careful examination of the quantitative results of the chromatogram. A failure to seal after the needle is withdrawn is more easily detected. The resulting "post-injection" leak is evident from variations in the characteristic retention time for a chromatographic peak resulting from variations in the column flow rate. Some capillary column injection ports may show different behavior for a post-injection leak because of their flow configuration.
Septum deterioration and resulting failure are serious problems. Monitoring a system for leaks can be costly and time consuming. Replacement of seals is inconvenient. Problems of detection and replacement are aggravated in automated systems which may run unattended for as many as 100 samples. A failure early in a run can impair the validity of the results for all subsequent samples.
Septum deterioration is inevitable due to the action of the syringe needles used for injection. Syringe needles must be strong enough to pierce the septa of sample containers and injection ports without bending or buckling. This strength requirement leads to the use of larger diameter needles. Larger diameter needles require greater insertion force to pierce a septum, which is thus subject to greater wear. The larger needles also make larger holes or tears in the septum, which are harder to reseal after needle withdrawal. Needles of smaller diameter cause less damage to the septum and make resealing easier, but are much more susceptible to bending when piercing a seal.
Syringe needles are made with sharp, beveled points to slice through the septa with lower force. However, the slicing action of repeated injections with beveled needles can lacerate a septum, which then begins to leak. In addition, small pieces of rubber torn from a septum by the needles can fall into the injection port liner. Once in the liner, these pieces can affect the analysis in two ways. First, they can release compounds which can appear as "ghost peaks" in the chromatogram. Second, they can adsorb or partition sample components as they pass through the injection port, causing distortion of peak shapes in the chromatogram. In either case, the validity of the resulting chromatogram is impaired.
Taking into account these considerations, most gas chromatographs use a rubber septum, approximately 3 mm thick and 6-12 mm in diameter. Syringes typically used with such septa are 10 microliter total capacity, with a sharp beveled 26 gauge, i.e., 0.48 mm diameter, needle. These are used for both manual injection and for automatic liquid samplers. Alternatively, thicker "cylindrical" septa are used to improve the reliability of sealing after multiple injections. These have the disadvantage of requiring higher syringe force.
Especially strenuous demands are made on a septum in automatic liquid samplers, such as the Hewlett-Packard 7673. This sampler uses a very rapid injection cycle. A differently shaped needle is adapted so that it can pierce a septum, the liquid sample can be injected into the injection port liner, and the needle can be removed from the port in less than 0.25 second before the syringe needle contents can be heated significantly by the injection port or the septum. The needle has a relatively large diameter, e.g., 0.66 mm, to allow it to withstand the higher force required to pierce the septum at high speed without bending or buckling. The needle has a blunt tip which facilitates a properly directed spray pattern for the sample. The larger diameter and blunt tip cause greater damage to the septum per injection, thus shortening the septum life before leaks occur or pieces of septum fall into the injection liner.
The problem of laceration can be addressed by using a septum having a predefined path for needle penetration. For example, septa have been adapted from "duckbill" seals. A duckbill seal comprises a flat rubber tube with two flat surfaces which can seal against each other. Duckbill seals are often used as check valves in flow systems because they open with very low pressure drop in one direction while sealing in the other direction. Duckbill seals are effective under high pressure, which causes the flat surfaces to press against each other more tightly.
Because a slit is preformed between the flat surfaces, a blunt needle can be inserted with low force through a duckbill seal many times without tearing or crumbling the rubber by forcing the flat surfaces apart. However, the flat seals do not form an effective seal about a syringe needle so leaks can occur during injection. In addition, the low pressure drops across the seal can be insufficient to close the flat surfaces, allowing post-injection leaks. The problem with post-injection leaks can be remedied by adding a spring to force the sealing surfaces together.
A duckbill seal has been included in a inlet assembly for a capillary column. For example, in the "Hewlett-Packard 1988 Analytical Supplies Catalog and Chromatography Reference Guide", page 35, a "Cool On-Column Inlet" includes a duckbill seal. Inspection of the actual system reveals that a stainless steel probe is used to separate the duckbill surfaces. Once the surfaces are separated by the probe, the needle is extended through the probe and the probe is withdrawn so that the duckbill closes around the needle. Neither the probe nor the duckbill seals the needle completely, so that some leakage generally occurs. It is noted that the inner perimeter of the duckbill seal is about 1.6 mm at the duckbill end and about 3.1 mm at the end where the needle is first inserted, so that this latter end never forms a seal with the needle.
A septum including interlocked syringe and duckbill seals is disclosed in U.S. Pat. No. 4,954,149 to Fullemann. The annular syringe seal prevents leakage during injection by syringe. The duckbill seal prevents fluid leakage after the needle is withdrawn from the septum. A spring clip is used to urge the duckbill closed as the needle is withdrawn to ensure closure even at low pressures. The syringe seal aperture is wider at the top to help guide the needle. This aperture converges to an annular base sealing element, the diameter of which is smaller than the needle to provide a tight seal while the needle is inserted. While a definite improvement over the simple duckbill seal, this hybrid-seal injection septum could leak due to deformation of the base sealing detail of the syringe seal when the needle was misaligned.
An improved interlocked injection septum uses dual annular sealing elements in the syringe seal. An annular sealing element is located at the base of the syringe seal, while an annular strain-relief element is located just above the sealing element. A misaligned needle deforms the guide detail. The deforming force, however, moves the sealing element into alignment with the needle. Thus, the sealing element is not deformed and so is able to form a secure seal against the needle.
This improved interlocked injection septum essentially answered the industry call for a reliable and longer lasting seal. However, once the original objectives were met, the industry raised its sights. After thousands of injections, the dual-detail interlocked injection septa are prone to leakage. What is needed is a similarly effective septum that has a substantially greater mean-time-before-failure (MTBF).